Example 2 Residual Stresses in Shot Peening

Shot peening is a cold working process in which thousands of spherical shot (typically '2 mm diam spheres of steel, glass, or ceramic) are propelled against the surface of a part. The overlap of the small craters formed by the impact of each sphere creates a compressive layer at the surface of the part. In this process, the intensity of the stresses generated typically exceeds half the yield strength of the material being peened. The depth of the residual stress layer obtained depends on the operating parameters of the process, including the intensity of the process as measured by Almen gages; the size, velocity, and impingement angle of the shot (Ref 41); and the characteristics of the material being shot peened.

Table 2 indicates the wide variation in fatigue life performance that can be obtained by varying the finishing process. These data clearly demonstrate the benefits of creating compressive residual stresses at the surface of a part by shot peening. In addition to improving the fatigue behavior of a part, controlled shot peening can also be used as a finishing process to generate three-dimensional surfaces such as those on aircraft wings and empennage skin. Additional examples of shot peening applications are listed in Ref 43. This process is described in detail in the article "Shot Peening" in this Volume.

Table 2 Fatigue life performance obtained by parts finished using various methods

Operation

Endurance limit in bending (107 cycles), MPa (ksi)

Relative endurance limit(a)

Surface grinding

Gentle

414(60)

100

Conventional

165(24)

40

Turning

Gentle

414(60)

100

Abusive

414(60)

100

Electrochemical machining

Standard

269(39)

65

Off standard

269(39)

65

Standard plus peening

538(78)

130

Off standard plus peening

462(67)

112

Electrical discharge machining

Finish

152(22)

37

Rough

152(22)

37

Finish plus peening

455(66)

110

Rough plus peening

517(75)

125

Electropolishing

Standard

290(42)

110

Standard plus peening

538(78)

130

Note: Material for all parts was Inconel 718. Source: Ref 42

(a) Expressed as a percentage of that obtained with gentle grinding.

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